JPH01135115A - Differential frequency detector - Google Patents

Abstract

PURPOSE: To form an automatic gain control circuit for a frequency difference detector by taking out automatic gain control signals from signals for which frequency multiplied signals are added and squared and supplying them to a variable gain amplification means. CONSTITUTION: Input to the automatic gain control circuit 40 is provided with the signals VI1 and VQ1 and the signals are respectively supplied through capacitors 44 and 46 to mixers 48 and 50 and frequency multiplied and converted into the intermediate frequency IF of 100KHz for instance supplied mutually in quadrature phase relation to the mixers in the mixers. The output of the mixers 48 and 50 is synthesized in an addition circuit 52. The synthetic signals are filtered in a band-pass filter 54 and the output signals of the filter are squared in a multiplier 56. A low-pass filter 58 is connected to the multiplier 56, ripples by modulation are eliminated by the low-pass filter and DC signals are taken out. The output of the low-pass filter 58 is supplied to an operational amplifier 60 and a reference voltage Vr is also supplied to the operational amplifier.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency difference detector with automatic gain control, and also to a carrier modulation receiver having a frequency difference detector.

In carrier modulated receivers, it is important to ensure that the local oscillator frequency tracks the transmitted carrier frequency and sets the difference between these two frequencies to within an acceptable accuracy range. An automatic frequency control (AFC) circuit is typically used to derive a value representing this frequency difference and automatically adjust the local oscillator. The frequency difference detector therefore forms the heart of any automatic frequency control loop and may be implemented in a variety of forms. The particular frequency difference detector relevant to the present invention is referred to as a balanced quadrature correlator. A known type of balanced quadrature correlator is
E.E. Transactions on Communications (IBBB Transactions)
on Communications)”, Vo.
l, COM 33. No. 2 No. 131-138
Page paper ``Properties of Frequency Detectors''
This known circuit has first and second mixers in quadrature relationship, in which the input signal is divided into two and four baseband signals. Frequency down-mixing is performed.In addition, low-pass filters are provided in the output circuits of the first and second mixers to select in-band signals.The output of one low-pass filter is [1] It is supplied to one input end of the third mixer, and (2) is differentiated and supplied to one input end of the fourth mixer.

The output of the other low-pass filter is (1) supplied to the other input terminal of the fourth mixer, and (2) differentiated and supplied to the other input terminal of the third mixer.

The outputs of the third and fourth mixers are subtracted from each other to produce a frequency difference signal. The output voltage Vd of such a balanced quadrature correlator is proportional to the frequency difference Δω between the transmitted carrier and the local oscillator and can be expressed as:

VdcCE2·Δω Here, E is the amplitude of the input signal. As is clear from this equation, the detected output voltage is highly sensitive to the amplitude of the input signal. This is a drawback of the circuit described above.

An automatic gain control circuit for a balanced quadrature correlator is known, for example, as described in Japanese Patent Application Laid-Open No. 137309/1983. In this known automatic gain control circuit, the signals originating from low-pass filters in two signal paths are fed to gain-controlled amplifiers, and the output signals of these amplifiers are squared and summed, thereby producing an unmodulated signal. generate a signal. This signal is fed to the gain control amplifier as a control signal.

A circuit with such a configuration is not optimal. The reason is that the multiplier used to square the output signal of the gain control amplifier will have dynamic range problems due to the DC offsets accumulated in the mixer and gain control amplifier.

It is an object of the present invention to provide an automatic gain control circuit for a frequency difference detector.

The present invention provides a frequency difference detector including means for producing first and second baseband signals in quadrature with respect to each other, and an in-band signal in the first and second baseband signals. an automatic gain control circuit comprising means for selecting a component, variable gain amplification means for adjusting the amplitude of an in-band signal component in response to an automatic gain control signal, and an automatic gain control circuit for producing said automatic gain control signal. are first and second input terminals capacitively coupled to a signal path carrying in-band signal components of said first and second signals in a quadrature relationship with each other, and the signals at said first and second input terminals, respectively. first and second mixers in quadrature phase relation to each other for frequency upconversion, means for summing these frequency uplifted signals, means for squaring the summed signal, and a means for squaring the summed signal; It is characterized by comprising means for extracting an automatic gain control signal and supplying it to the variable gain amplification means.

The present invention also provides a frequency difference detector, wherein the frequency difference detector includes a signal input terminal, a local oscillator, first and second mixers, and a signal input terminal connected to the first and second mixer. means for coupling said local oscillator to a first input of said mixer to a second input of said first and second mixers; 9 in the signal path or local oscillator path leading to the mixer such that the first and second mixers produce first and second outputs, respectively, in quadrature with respect to each other;
a 0° phase shifter, first and second frequency selection means provided in the output circuits of the first and second mixers, respectively, and a first frequency selection means provided in the output circuits of the first and second mixers; a first electrical path connecting the first frequency selection means to the second input end of the fourth mixer; and a second electric path connecting the second frequency selection means to the fourth mixer. first vessel
a third electrical path connecting the second frequency selection means to the second input end of the third mixer; and a fourth electrical path connecting the second frequency selection means to the second input end of the third mixer; a differentiating means provided in the passage; and said third and fourth
signal subtraction means connected to the output of the mixer; first and second signal subtraction means connected respectively in first and second signal branches between said input terminal and said third and fourth mixers; comprising an adjustable gain amplifier and an automatic gain control circuit;
The automatic gain control circuit has a fifth and a sixth mixer;
The first input terminals of these fifth and sixth mixers are connected to the first input terminals of the fifth and sixth mixers.
and a second signal branch, the second inputs of the fifth and sixth mixers being respectively connected to mutually quadrature outputs of the intermediate frequency local oscillator, the automatic gain control circuit Furthermore, addition means for adding the outputs of the fifth and sixth mixers, signal squaring means coupled to the addition means, and low-pass filter means for passing at least the DC component of the output signal of the signal squaring means. and the DC component has a gain control signal for the first and second adjustable gain amplifiers.

In a frequency difference detector constructed in accordance with the present invention, the use of capacitors minimizes the problem of DC offset in the signal supplied to the automatic gain control circuit. Furthermore, unlike known automatic gain control circuits that square the baseband signal, the automatic gain control signal is derived from the remodulated signal at the intermediate frequency. Also, the design of the fifth and sixth mixers used for orthogonal remodulation of the baseband signal is not critical. The reason for this is that the signal obtained by adding the outputs of these mixers is used only to obtain a measure of the input signal strength, and is not used for remodulation.

If desired, a bandpass filter can be connected between the summing means for summing the outputs of the fifth and sixth mixers and the signal squaring means. The envelope of the signal at the output of this bandpass filter represents the input signal amplitude E.

The invention will be explained with reference to the drawings.

Corresponding elements in each figure are given the same reference numerals.

The frequency difference detector according to the invention shown in FIG. It is connected. These mixers12.14
The outputs of the mixer 14 are in a quadrature relationship with each other, and in this example, this quadrature relationship is obtained by phase shifting the local oscillator signal provided to the second mixer 14. Or input terminal 1
0 and the signal input terminal of the first or second mixer.
A phase shifter can also be connected. The frequency of the local oscillator can be the same as the carrier frequency of the input signal V1 or can be slightly different. Low pass filter 18, 2
0 selects the difference component in the outputs of the first and second mixers 12.14. The amplitude of the filtered signal is adjusted by an amplifier 22.24 with adjustable gain (amplification) to produce signals Vll and VQI. Here ■ means in-phase, and Q means quadrature phase. These signals Vll and VQI are once differentiated by differentiating circuits 26 and 28, respectively, and are supplied to the first input terminals of a third mixer 30 and a fourth mixer 32 as signals VI2 and VO2, respectively. These signals VI2 and VO2 are differentiated by differentiating circuits 34 and 36, respectively, and are supplied to the second input terminals of the fourth mixer 32 and the third mixer 30 as signals Vl'1 and VQ' l , respectively. The outputs Vbi and VQ' of mixers 30 and 32 are supplied to a subtraction circuit 38 which outputs an output signal V. occurs.

By differentiating the signals applied to both the third and fourth mixer inputs, any DC offset voltages are rejected. The reason is that the "differential" is a high-pass function. Therefore, the dynamic range of these mixers 30, 32 is not affected by the offsets accumulated in these previous circuit stages, which appear in the output signal V. The mixer 30 removes the offset voltage of only the DC component that may be
．． 32 o.

and the subtraction circuit 38. mixer 30.32
There should be a first-order difference between the differential signals applied to the first inputs of the mixers and the second inputs of these mixers.

The signals at the various circuit points in FIG. 1 are as follows: It is.

V1=E-cos(ω,・t) Vll=-・cos(Δω°t) VI2=-first Δω-5in(Δω・t)・5in(
Δω・t)] VQl = −・5in(△ω °t)巳VQ2=−9Δω・cos(Δω・t)・5in(Δω
・1)) VB-1 [Δω・5ln(Δω・t)・C08(Δω
・t)・vO“−1: [Δω・5ln(Δω・t)・C
08(Δω・t)・Va = VI'VQ' −
-.(Δω)3 CCB2(Δω)3 where E is the amplitude and Δω is the difference between the transmit carrier frequency ω and the local oscillator frequency ω, resulting in Δω−(ω, −ω,). Vo
The last equation above for represents that the output of the frequency difference detector is proportional to (Δω)3 rather than (Δω) as in the case of a normal balanced quadrature correlator.

Compared to a given change in frequency difference (Δω), this change in the output of the embodiment of FIG. 1 is much larger, which is a desirable characteristic. Such characteristics apply to the third and fourth mixers 30.3
2 and subtraction circuit 38 to reduce residual problems of DC offset.

Assuming there is a first-order difference in differential between the first and second input terminals of each of the third and fourth mixers 30.32, there is no DC offset at the inputs to the third and fourth mixers 30.32. In order to achieve this, differentiating circuits can be arranged in various ways. A limiting factor in choosing the placement of the differentiator circuit is that differentiation is a noise enhancement process, and in practice a compromise must be made between circuit complexity and noise considerations.

The output signal Vo of subtraction circuit 38 (FIG. 1) includes a term designated E2. Here E is the input voltage to the frequency difference detector. Output signal V. In order to make the frequency difference detector less sensitive to variations in the input signal V1, the frequency difference detector shown in FIG. 1 is provided with an automatic gain control (AGC) circuit 40. Adjustable gain amplifiers 22 and 24 are provided in the I and Q signal paths at appropriate signal locations. The points shown at the outputs of the low-pass filters 18 and 20 are only examples for illustrating the operating principle.

The inputs to automatic gain control circuit 40 are signals H1 and V
QI and these signals are connected to capacitors 44 and 46.
to mixers 48 and 50, respectively, where it is frequency upconverted to an intermediate frequency (IF) of, for example, 100 KHz, which is fed to the mixers in quadrature with respect to each other. The outputs of mixers 48 and 50 are combined in adder circuit 52. This composite signal is filtered by a bandpass filter 54, and the output signal of this filter is squared by a multiplier 56. A low-pass filter 58 is connected to this multiplier 56, and this low-pass filter removes ripples caused by modulation and extracts a DC signal. The output of this low-pass filter 58 is fed to an operational amplifier 60.
A reference voltage Vr is also supplied to this operational amplifier. The output voltage of operational amplifier 60 has a DC gain control signal for amplifier 22.24.

Squaring of the signal resulting from bandpass filter 54 can also be performed using a rectifier or a logarithmic amplifier.

The frequency difference detector according to the invention can be used as a demodulator for FM signals. However, the output signal V of the frequency difference detector. Since V is not linearly related to the frequency difference of the input signal V, but is proportional to (Δω)3, an additional signal processing step is required to recover the original modulated signal.

Frequency difference detector output V. The equation for can be rewritten as the following equation.

Vo=K(Δω)3=K (ωc + ω,)here ω. is a constant frequency difference, ω0 is the instantaneous frequency produced by the modulating signal, and K is the amplification constant.

V in the above formula. A desired modulated signal can be reproduced by processing with a cubic function.

This can be done digitally or by analog signal processing. However, there is no easy function to realize the cube function. A digital example will be explained with reference to FIG.

The output signal V of the frequency difference detector. is the low pass filter 62
an analog-to-digital converter 64 clocked by a clock signal on line 65;
is supplied to The digitized output of this converter is
A read-only note that serves as a lookup table for the desired cubic function, i.e.! J (ROM)66. This ROM
The output of 66 is provided to a digital to analog converter 68 which produces an analog cube root signal which is filtered by a low pass filter 70 to provide an output comprising a modulated signal.

Using the frequency difference detector described above as an FM demodulator provides two advantages over conventional dual branch architectures in reducing output distortion:

(1) The problem of holes in the frequency spectrum caused by AC coupling is eliminated.

(2) Cross-coupling of the two branches cancels out the gain and phase shifts of these two branches, thus reducing distortions and disturbances in the audio output normally caused by these imperfections.

Another example of a frequency difference detector that performs automatic gain control is shown in FIG. The basic frequency difference detector itself without automatic gain control circuit 40 is known, for example, in ``IEE Transactions'' published in February 1985.
On Communications (IB8B Transa
tions on Communications
)”.

Vol, COM 33. No. 2 No. 131-13
The 8-page paper “Properties on Frequency Difference Detectors”
Es of Frequency Difference
The difference between the example shown in FIG. 3 and the example shown in FIG. 1 is that in the example shown in FIG. .32
The fact that the signals input to the first input of the frequency difference detector are not differentiated, i.e. not high-pass filtered, and therefore contain DC offsets accumulated in previous processing steps in the frequency difference detector be.

Output signal V. Since contains the term denoted E2, this output signal is sensitive to amplitude changes in the human input signal. The provision of automatic gain control circuit 40 reduces the sensitivity of this output signal to amplitude changes in human input signal V1. This automatic gain control circuit has been fully described with reference to FIG. 1 and will not be described again here.

The invention is not limited to the embodiments described above, but can of course be modified in many ways. Other features known in the design, manufacture, and use of frequency difference detectors and carrier modulation receivers and their components may be included in the variations described above in addition to or in place of the features described above.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram showing an example of a frequency difference detector according to the present invention, FIG. 2 is a circuit diagram showing a digital demodulator, and FIG. 3 is a block circuit diagram showing another example of a frequency difference detector according to the present invention. FIG. 10... Input terminal 12... First mixing stage 1
4... Second mixing stage 16... Local oscillator 18
, 20.58.62.70...Low pass filter 22
, 24...Amplifier 26, 28.34.36...Differentiating circuit 30...Third
Mixer 32...Fourth mixer 38...Subtraction circuit 40...Automatic gain control circuit 48,5.
0...Mixer 52...Addition circuit 54...
Bandpass filter 56... Multiplier 60... Operational amplifier 64... Analog-to-digital converter 66... Read-only memory 68... Digital-to-analog converter patent applicant NV Philips Fluor ilanpenfabriken

Claims (1)

[Claims] 1. In a frequency difference detector, this frequency difference detector:
means for producing first and second baseband signals in quadrature with respect to each other; means for selecting in-band signal components in the first and second baseband signals; and means for selecting in-band signal components in the first and second baseband signals; variable gain amplification means for adjusting the amplitude of the component; and automatic gain control circuitry for producing said automatic gain control signal, said automatic gain control circuitry comprising a band of said first and second signals in quadrature with respect to each other. The first and second channels are capacitively coupled to a signal path through which internal signal components flow.
an input terminal and a first circuit in quadrature with respect to each other for frequency up-converting the signals at the first and second input terminals, respectively;
and a second mixer, means for adding these frequency-stepped signals, means for squaring the summed signal, and extracting an automatic gain control signal from the squared signal to perform the variable gain amplification. A frequency difference detector comprising means for supplying the frequency difference detector to the frequency difference detector. 2. In the frequency difference detector, this frequency difference detector is
a signal input terminal, a local oscillator, first and second mixers; the signal input terminal is coupled to the first input terminals of the first and second mixers; and means for coupling to a second input of a second mixer, in a signal path or local oscillator path leading to one of the first and second mixers; and a 90° phase shifter for causing the second mixer to produce first and second outputs, respectively, in quadrature with respect to each other; a first electrical path connecting said first frequency selecting means to a first input of a third mixer; and a first electrical path connecting said first frequency selecting means to a second input of a fourth mixer. a second electrical path connecting the second frequency selection means to the first input end of the fourth mixer; a third electric path connecting the second frequency selection means to the first input end of the fourth mixer; a fourth electrical path connected to the second input end of the three mixers; differentiating means provided in the second and fourth electrical paths; and a fourth electrical path connected to the output ends of the third and fourth mixers. first and second adjustable gain amplifiers connected in first and second signal branches, respectively, between said input terminal and said third and fourth mixers;
an automatic gain control circuit, the automatic gain control circuit having fifth and sixth mixers, first inputs of the fifth and sixth mixers respectively connected to the first and second signal branches. The second inputs of the fifth and sixth mixers are capacitively coupled and are respectively connected to mutually quadrature outputs of the intermediate frequency local oscillator, the automatic gain control circuit further comprising: It has an addition means for adding the outputs of the sixth mixer, a signal squaring means coupled to the addition means, and a low-pass filter means for passing at least the DC component of the output signal of the signal squaring means, A frequency difference detector characterized in that the DC component comprises a gain control signal for the first and second adjustable gain amplifiers. 3. The frequency difference detector according to claim 2, wherein a bandpass filter is connected between the adding means and the signal squaring means. 4. The frequency difference detector according to claim 2 or 3, wherein the first and third electrical paths are provided with differentiating means, and the differentiating means in the first and third electrical paths are provided in the second and fourth electrical paths. A frequency difference detector characterized in that it differs by one order compared to the differentiating means in the electrical path. 5. A frequency difference detector according to claim 4, wherein at least one differentiating means in the first and second electrical paths is common to both the first and second electrical paths;
A frequency difference detector, characterized in that at least one differentiating means in said third and fourth electrical paths is common to both said third and fourth electrical paths. 6. A carrier modulation receiver comprising an FM demodulator including the frequency difference detector according to any one of claims 1 to 5. 7. In the carrier modulation receiver according to claim 6, the output signal of the frequency difference detector has a cubic function, and the modulated signal is reproduced by obtaining the cube root of this cubic function. A carrier modulation receiver featuring: 8. In the carrier modulation receiver according to claim 7, the output of the frequency difference detector is supplied to a digital cube root circuit, and the digital cube root circuit is configured to calculate the third order of the cubic function. an analog-to-digital converter having an output connected to a look-up table containing multiplicative roots; and a digital-to-analog converter coupled to said look-up table and having an output for a modulated signal. A carrier modulation receiver featuring: